Perinatal women with co-occurring substance use disorder and major depressive disorder (SUD-MDD) are of public health concern because they face numerous neonatal and obstetrical risks in addition to the emotional distress, impairment, and adverse health effects associated with these disorders. Despite the serious morbidity associated with both SUD and MDD in perinatal women and despite the need for population-specific treatments that address the unique set of challenges associated with this period of a women's life, there are virtually no empirically-based treatments that target the specific needs of perinatal women with SUD-MDD. In this treatment development grant, we propose to pilot test a novel 12-week individual treatment, Sober Network IPT for perinatal women with SUD-MDD, which focuses on network support strategies for SUD within a broader Interpersonal Psychotherapy (IPT) framework; the treatment of choice for perinatal MDD. Sober network support is theoretically consistent with IPT and has been identified as an empirically supported mechanism of change of many efficacious SUD treatments. The purposes of this treatment development grant are to (a) integrate network support strategies for SUD into an IPT framework to create Sober Network IPT for perinatal women with SUD-MDD, and (b) conduct a randomized pilot trial in a sample of 50 women who meet criteria for SUD-MDD during pregnancy or within 3 months postpartum to demonstrate the feasibility and acceptability of the proposed recruitment methods and research design, of the therapist training methods and of delivering the Sober Network IPT treatment. We will evaluate the extent to which Sober Network IPT relative to Enhanced Dose-Matched Treatment as Usual will result in the following outcomes: fewer heavy drinking/drug using days through the 3 month follow-up, reduced depressive symptoms at post-treatment and 3 month follow-up (primary) and improved sober support and social support (secondary). We will explore the feasibility of using fetal, neonatal and infant outcomes and engagement in health prevention as secondary outcomes in a subsequent trial. As a result of this R34 Exploratory Research Project, we will have adapted IPT into Sober Network IPT, a novel treatment approach for perinatal women with SUD-MDD, tested its feasibility, acceptability, and initial efficacy, which will lay the groundwork future R01 clinical trials. Because our proposed research will be conducted in a clinical setting in collaboration with community therapists/providers, and with patients with multiple problems and a range of clinical difficulties, our proposed program of research, from a longer term perspective, will inform the clinical care of perinatal women with SUD-MDD and has the potential for dissemination to other community agencies that treat perinatal women with SUD-MDD; underserved women with clinical concerns that are of great public health significance. Furthermore, if Sober Network IPT is found to be efficacious in our vulnerable target population, we anticipate that it could hold promise for a more heterogeneous SUD-MDD population.